Neuropeptides of the vasopressin-oxytocin family play important roles in both affiliative and asocial behaviors across a broad range of vertebrate taxa. Similarly, recent findings in human clinical populations (personality disorder, autism, depression) indicate that impairments of sociality may be produced at least in part by dysfunctions of these peptide systems. The relevance of comparative data for human populations is further underscored by the observation that across vertebrate groups, including humans, behavioral functions and major features of vasopressin-oxytocin brain distributions are conserved. Nonetheless, vasopressin-oxytocin systems exhibit remarkably plastic features as well. A variety of data now suggest the hypothesis advanced here that the expression of specific and integrated patterns of behavior (e.g., behavior typical or specific to an individual's sex, social tactics, and species' social structure) is produced in part by the functional lability of vasopressin-oxytocin systems. However, few experiments have explicitly tested for sex differences in the modulation of similar behaviors, and in most species, only a limited range of social behaviors have been examined. Proposed experiments will employ intracerebroventricular infusions of vasopressin- and oxytocin-like peptides in both male and female songbirds to determine neuropeptide functions for a broad , range of behaviors, including aggression, courtship, sexual behavior, partner preference (typical of monogamous pair-bonds), and non-sexual affiliation. The diversity of social structures in songbirds allows specific aspects of behavior to be examined while holding potential confounds constant; our previous experiments have shown that neuropeptides modulate male aggression differently in colonial vs. territorial songbird species (i.e., highly gregarious vs. relatively asocial). Experiments proposed here extend these species comparisons to females and to a broader range of social behaviors. Finally, neuroanatomical techniques (immunocytochemistry for immediate early gene expression and peptides; quantitative autoradiography) will be used to establish neural bases for the previously-identified divergence in peptide function between affiliative and asocial species.